The period between optimal function and reproductive decline is being investigated in female golden hamsters. The current emphasis is on basis processes in uterine function that may show changes as the females age. Biochemical studies include methods to determine uptake of steroids and receptor levels in endometrium and myometrium. Metabolism of the steroids and various parameters of uterine tissue response are being examined biochemically and histochemically. Rate of response is of particular concern. In addition to uterine changes, ovarian function is also of concern. Blood levels of steroids are measured and their relation to implantation and uterine response correlated. Histochemistry of the ovary includes studies of enzymes, lipids, cell types and innervation. Studies on ovulation and early ovum maturation are included in attempts to correlate with the other reproductive events.